Proton range monitoring with in-beam PET: Monte Carlo activity predictions and comparison with cyclotron data

Goal Proton treatment monitoring with Positron-Emission-Tomography (PET) is based on comparing measured and Monte Carlo (MC) predicted β+ activity distributions. Here we present PET β+ activity data and MC predictions both during and after proton irradiation of homogeneous PMMA targets, where protons were extracted from a cyclotron. Methods and materials PMMA phantoms were irradiated with 62 MeV protons extracted from the CATANA cyclotron. PET activity data were acquired with a 10 × 10 cm2 planar PET system and compared with predictions from the FLUKA MC generator. We investigated which isotopes are produced and decay during irradiation, and compared them to the situation after irradiation. For various irradiation conditions we compared one-dimensional activity distributions of MC and data, focussing on Δw50%, i.e., the distance between the 50% rise and 50% fall-off position. Results The PET system is able to acquire data during and after cyclotron irradiation. For PMMA phantoms the difference between the FLUKA MC prediction and our data in Δw50% is less than 1 mm. The ratio of PET activity events during and after irradiation is about 1 in both data and FLUKA, when equal time-frames are considered. Some differences are observed in profile shape. Conclusion We found a good agreement in Δw50% and in the ratio between beam-on and beam-off activity between the PET data and the FLUKA MC predictions in all irradiation conditions.